Various processes for the preparation of carboxylic acid anhydrides are known. For example, acetic anhydride, the most common anhydride, may be prepared from acetic acid by the steps of (1) cracking or pyrolyzing acetic acid to produce ketene and (2) reacting the ketene with acetic acid to produce acetic anhydride. Higher molecular weight carboxylic acid anhydrides, e.g., substituted anhydrides and/or anhydrides which contain more than 4 carbon atoms, typically are prepared by contacting the corresponding carboxylic acid with acetic anhydride, e.g., butyric anhydride may be prepared by contacting butyric acid with acetic anhydride.
Trifluoroacetic anhydride (TFAA) is a strong dehydrating agent and has a broad range of chemical reactivity including the activation of carboxylic acids as mixed anhydrides (J. M. Tedder Chem. Rev. 1955, 55, 787-827). TFAA is a useful chemical in the synthesis of polymers and fine chemicals. However, it is expensive and, thus, more efficient methods for its synthesis are desirable. Because TFAA is a very reactive anhydride, a strong desiccant is required for its preparation. It was first prepared by Swarts in 1922 (Bull. Sci. Acad. Roy. Belg. 1922, 8, 343-70) by the dehydration of TFA using phosphorus pentoxide. This method is convenient for small-scale TFAA synthesis but too inefficient for large-scale production. Phosphorus pentoxide is a water-sensitive solid that is difficult to work with on a large scale. Its cost and the expenses associated with the large amount of phosphate-containing waste it generates are strong disadvantages. In addition, minimizing manufacturing costs by minimizing waste is highly desirable.
The use of sulfur trioxide as the desiccant is an improvement over phosphorus pentoxide in this respect. TFM can be produced by the reaction of trifluoroacetyl chloride and sodium trifluoroacetate with the coproduction of sodium chloride which is less difficult to dispose of than is phosphoric acid or sulfuric acid resulting from the phosphorus pentoxide and sulfur trioxide processes. However, the cost of producing trifluoroacetyl chloride limits the feasibility of this method.
In 1954, E. J. Bourne and coworkers, J. Chem. Soc. 1954, 2006-12, showed that at equilibrium the reaction of acetic anhydride and TFA to produce Ac-TFA and the reaction of Ac-TFA and TFA to produce TFM are not favorable. Nonetheless, the production of TFAA by reacting Ac.sub.2 O with TFA is described in U.S. Pat. No. 4,595,541 which discloses a process for the preparation of TFAA by contacting TFA with the anhydride of acetic or an .alpha.-halogenated carboxylic acid. Thus, contacting TFA (8 molar equivalents) with acetic, mono-, di- or tri-chloroacetic anhydrides produces TFAA in yields of 36, 59, 67 and 74%, respectively, and the conclusion that anhydride-based processes using a-chlorinated acetic anhydrides are preferred for the synthesis of TFAA. A process that produces TFAA in high yields from inexpensive raw materials and generates little or no waste which presents disposal problems, e.g., only water or marketable byproducts, is thus highly desirable. The present invention provides such a process.